Pick a basis and bit for Alice. The same Alice photon is sent into all four cases below.
Eve guesses one of two bases (she doesn't know Alice's). Bob also picks a basis. There are four combinations:
Each replay re-rolls the random projections. Bars track how many times each detector fired across all replays of the current case.
When Eve's basis matches Alice's, her measurement is deterministic and Bob's outcome (when his basis also matches) is correct. When Eve's basis disagrees with Alice's, her measurement is random — and that's what corrupts the bit.